Automatic information reading from bank credit cards, transportation fare cards, and like objects is rapidly gaining acceptance as the society moves towards a "cashless" money system. The information is usually placed, visibly or invisibly, on an article and read by mechanical means, by light-detection means or by magnetic-sensing means. For example, the so-called magnetic-stripe credit cards or fare cards are particularly attractive because of their ease of manufacture. Unfortunately, this type of card has been demonstrated to be easily counterfeited. Despite this known fact, the convenience of the "magnetic stripe" approach has caused the American Bank Association to pick the magnetic stripe as its standard for bank card credit plans.
In other magnetic recording systems, it is also desirable to have some means of building more security into the system. For example, knowledge of a code key would be useful in screening access to information stored on magnetic tape. This code key might be mechanical or electrical according to known procedures. But, such systems would not adequately discriminate between persons who are allowed access to some tapes but not allowed to retrieve information on other tapes. It would be convenient if each package of information, say each magnetic tape in a given library, held a code signal which had to be recognized by the retriever before being erased to reveal the primary information on the tape. It would be further convenient if the code could be changed from time to time as the authorized personnel change.
In general then, it would be convenient to provide magnetic recording media capable of carrying two indpendent signals in the same recording area.
In a hindsight review of art bearing some relationship to fields relating to the invention, the following art was located:
U.S. Pat. No. 3,601,913 to Pollock suggests a device utilizing a mixture of high and low coercivity materials. The utility of this system depends on the detection of magnetic voids caused by mechanical displacement of magnetic powder bearing surfaces. Pollock also suggests that use of such mixtures, will make card counterfeiting more difficult. But Pollock uses his mixture only to complicate the manipulative acts of duplicating a card; the individual components serve no distinct functions except in instances wherein they are at least partly non-congruent so that they yield a visually-identifiable pattern or a geometrically-distinct identifying pattern, formed of one of the magnetic powders. In general, the powders simultaneously respond to the field as a single-population powder would.
Moreover, the chromium oxide and iron oxide mixtures of the Pollock patent would be inoperable in forming combinations for use in such processes as are to be described below.
In recently-issued U.S. Pat. No. 3,761,311 to Perrington et al, there is described a dual layer magnetic tape, each layer having different nominal coercivities. It is believed that such a tape is sold under the trade designation Scotch Brand C-60 Cassette Cobalt-Energized High-Energy Type (Catalog Number S-C-60ME) by 3M Company. That tape is not bimodal, probably because magnetic interaction substantially prevades the ultra-thin layer of the tape. Moreover, since it was developed for, and is sold to, a specific audio recording market, it is clearly not intended for any bimodal use.
In the following specifications the term "card" will be used in the sense of a mechanical equivalent of any article which carries magnetic material in information-yielding arrangement. It will be obvious that the scope of the kinds of members which can be tagged with magnetic identification means is very broad and that such recording members are mechanical equivalents to the credit card referred to in this application.